Cholesterol absorption and sterol balance studies have been initiated in normal subjects and in patients with hyperlipidemia without diabetes mellitus and in diabetic patients with and without hyperlipidemia. The following additional measurements are being carried out in these study subjects: fasting and postprandial plasma glucose, serum cholesterol and triglycerides, quantitative lipoprotein lipid determinations, fasting plasma insulin, glucagon and gastric inhibitory polypeptide, glucose tolerance testing in nondiabetic subjects with glucose, insulin, glucagon and gastric inhibitory polypeptide measurements after the glucose challenge, quantitative serum and urine ketones and urinary nitrogen. A minimum of 60 subjects will be enrolled in this study: 10 normal control subjects, 10 patients with hypercholesterolemia without diabetes mellitus, 10 patients with hypertriglyceridemia without diabetes mellitus, 10 diabetic patients without hyperlipidemia, 10 diabetic patients with hypercholesterolemia and 10 diabetic patients with hypertriglyceridemia. To date, 51 subjects have completed their studies. A preliminary analysis of the data indicates differences in cholesterol absorption and sterol excretion in the groups of subjects studied. Cholesterol absorption is decreased in diabetic patients with hypertriglyceridemia and appears to be negatively correlated with neutral sterol excretion in diabetic and nondiabetic subjects with normal lipids. Neutral sterol excretion is increased in patients with hypertriglyceridemia in diabetic and non-diabetic subjects and is negatively correlated with HDL-cholesterol in hypertriglyceridemia in diabetic and nondiabetic subjects and in negatively correlated with HDL-cholesterol in hypertriglyceridemic subjects. Bile acid excretion tends to be lower in diabetic versus nondiabetic, is decreased in patients with hypercholesterolemia whether diabetic or nondiabetic and is increased in patients with hypertriglyceridemia. The estimate of cholesterol conversion to bile acids (percent of sterol excretion represented by bile acids) is lower in patients with hyperlipidemia. The estimated cholesterol synthesis (total sterol output minus dietary cholesterol) is increased in patients with hypertriglyceridemia whether diabetic or nondiabetic. Such varations in cholesterol absorption and sterol excretion may provide a mechanism for accelerated atherogenesis in diabetic patients with and without hyperlipidemia.